1. Field of the Invention
The present Invention relates to a wavelength-variable semiconductor laser light source for optical coherent communication, which varies the oscillation wavelength of the semiconductor laser while continuing the phase of oscillated light.
2. Background Art
FIG. 5 is a block diagram showing a conventional wavelength-variable semiconductor laser light source. In FIG. 5, reference numeral 1 indicates a semiconductor laser, reference numeral 1A indicates an antireflection film, reference numerals 2A and 2B indicate lenses, reference numeral 5 indicates a diffraction grating, reference numeral 6 indicates a rotating stage, reference numeral 7 indicates a parallel sliding stage, reference numeral 8 indicates an arm, reference numeral 9 indicates a fixed plate, reference numeral 11 indicates a wavelength setting part, reference numeral 12 indicates a comparator, reference numeral 13 indicates a parallel sliding driver, and reference numeral 16 indicates a displacement gauge.
In the arrangement of FIG. 5, one end face of semiconductor laser 1 is coated with antireflection film 1A. From the end face with the antireflection film, outgoing light 3B is outputted. The outgoing light 3B is transformed into collimated light by lens 2B and is incident on the central part of diffraction grating 5. At this time, the central part of the diffraction grating 5 and the other end face without an antireflection film of the semiconductor laser 1 form an external resonator. Semiconductor laser 1 oscillates at a single mode and outputs outgoing light 3A from the other end Face.
Here, diffraction grating 5 is fixed on rotating stage 6 and the rotating stage 6 is provided on parallel sliding stage 7 which moves in parallel with the optical axis of semiconductor laser 1. Furthermore, the rotating stage 6 is in contact with fixed plate 9 via arm 8. Therefore, arm 8 slides on the plane of fixed plate 9, whereby the parallel motion of parallel sliding stage 7 is transformed into the rotational motion of rotating stage 6; thus, the oscillation wavelength of semiconductor 1 is varied under phase-continuous conditions by way of the parallel movement of the parallel sliding stage 7.
Wavelength setting part 11 sets up the oscillation wavelength of semiconductor 1. Displacement gauge 17 detects the amount of the parallel displacement of parallel sliding stage 7 and outputs a displacement signal which corresponds to the amount of the parallel displacement. Comparator 10 compares a set signal from wavelength setting part 9 and the displacement signal from displacement gauge 16 and outputs a control signal to parallel sliding driver 13 in accordance with the result of the comparison. In this way, the oscillation wavelength of semiconductor laser 1 is set arbitrarily within the resolution of the displacement gauge under phase-continuous conditions.
On the other hand, outgoing light 3A from the other end face without an antireflection film of semiconductor laser 1 is transformed into collimated light via lens 2A; and the collimated light becomes an output light from the wavelength-variable semiconductor laser light source.
The set resolution of the oscillation wavelength of the semiconductor laser 1 is limited by the resolution of the rotation angle of the diffraction grating which acts as an external mirror of the semiconductor laser; thus, in order to raise the set resolution of the oscillation wavelength, it is necessary to raise the resolution of the rotation angle of the diffraction grating.
However, in the arrangement of FIG. 5, the rotation angle of the diffraction grating 5 is calculated in accordance with the amount of the parallel displacement of the parallel sliding stage 7; thus, the set resolution of the oscillation wavelength of the semiconductor laser is limited by the resolution of the displacement gauge 16 which detects the amount of the parallel displacement of the parallel sliding stage.
The relationship between quantity .DELTA.X of the parallel displacement of parallel sliding stage 7 and oscillation wavelength .lambda. of the semiconductor laser 1 is represented by the following formula, in which d is the interval of the grooves of the diffraction grating, .theta. is the initial angle of tile diffraction grating, .DELTA..theta. is the angle of the rotation of the diffraction grating, L is the initial length of the external resonator, and m is the order of the oscillation mode. EQU .lambda.=2.multidot.d.multidot.sin(.theta.+.DELTA..theta.)=2(L+.DELTA.X)/m
FIG. 4 shows an example result of the calculation in accordance with the above formula, in which interval d of the grooves of the diffraction grating is 1/1200 mm, initial angle .theta. is 68.4.degree., initial length L of the external resonator is 36.27 mm, order m of the oscillation mode is 46800. According to FIG. 4, a displacement gauge which can detect the amount of the parallel displacement of the parallel sliding stage 7 with the resolution of 10 nm or less is needed in order to make the set resolution of the oscillation wavelength of semiconductor laser 1 to be 0.4 pm (picometer).
However, the resolution of the presently-obtainable displacement gauge regarded as one having the highest resolution, which gauge is of a strain gauge-type or a differential transformer-type, is 20 nm at best. Therefore, a problem occurs in that the oscillation wavelength of the semiconductor laser 1 cannot be set up with a resolution of 0.8 pm or less, the value (0.8 pm) corresponding to the resolution (20 nm) of the displacement gauge 16.
On the other hand, the displacement gauge based on an interference method by using, for example, a Michelson interferometer can have a resolution of 20 nm or less. However, in this case, the system for constructing the gauge is complicated; thus, it is not practical to use this type of gauge for the wavelength-variable semiconductor laser light source in which the oscillation wavelength of the semiconductor laser is set with high resolution under phase-continuous conditions.